In the field of semiconductor processing, chemical-mechanical polishing (CMP) is a widely used technique for planarization of material and controlled removal of a layer of material from a stack of films on a substrate. In a typical CMP process, a film is selectively removed from a semiconductor wafer by rotating the wafer against a polishing pad (or moving the pad against the wafer, or both) with a controlled amount of pressure in the presence of a slurry. FIG. 1 shows a typical CMP apparatus 10 in which a workpiece 100 (such as a silicon wafer with one or more layers deposited thereon) is held face down by a wafer carrier 11 and polished using a polishing pad 12 located on a polishing table 13; the workpiece is in contact with slurry 14. The wafer carrier 11 is rotated by a shaft 15 driven by a motor 16. The entire surface of the workpiece is polished by the polishing pad in the presence of the slurry. Accordingly, surface irregularities are removed from the films deposited on the substrate, and a high degree of planarization is obtained. CMP has been used to remove and/or planarize a wide variety of materials from a stack of films on silicon substrates, including polysilicon, silicon oxides and silicon nitride.
More recently, CMP has been used in a cloisonne process in the fabrication of a magnetic read/write head. This process involves polishing of an aluminum oxide (Al.sub.2 O.sub.3) film. As shown in FIG. 2A, a layer 22 of aluminum oxide is deposited over small structures 21 of NiFe; the NiFe structures 21 are disposed on an underlying structure 1, which may be a substrate or a stack of films. The aluminum oxide layer 22 is then planarized and removed by CMP until the top surface 21a of each NiFe structure 21 is exposed (see FIG. 2B). A cloisonne pattern is thus obtained, with the top surface of the Al.sub.2 O.sub.3 layer 22 coplanar with the top surface of the NiFe structures 21.
In CMP processes generally, it is extremely important to stop the process at a desired predetermined location in the film or stack of films (that is, when the endpoint has been reached). Overpolishing (removing too much) of a film renders the workpiece unusable for further processing, thereby resulting in yield loss. Underpolishing (removing too little) of the film requires that the CMP process be repeated, which is tedious and costly. Underpolishing may sometimes go unnoticed, which also results in yield loss. In the above-described cloisonne process, it is particularly important to maintain tight tolerances on the thickness of structures 21, while assuring that enough of the aluminum oxide layer is removed to expose surface 21a.
In a conventional approach to the CMP endpoint detection problem, the thickness of the layer to be removed and the polishing rate are measured for each workpiece, in order to determine a desired polishing time. The CMP process is simply run for this length of time, and then stopped. Since many different factors influence the polishing rate, and the polishing rate itself can change during a process, this approach is far from satisfactory. In particular, the polishing rate of a film generally changes substantially near an interface; this further compounds the problem of predicting the desired polishing time.
Furthermore, as shown in FIG. 2B, the desired cloisonne structure has a very small pattern factor; that is, polishing of the aluminum oxide layer 22 must continue until the NiFe surface 21a is exposed, but the total exposed NiFe area is only about 2% of the total Al.sub.2 O.sub.3 /NiFe interface area.
Application of CMP processing to head technology therefore requires a CMP endpoint detection technique which is effective with materials such as Al.sub.2 O.sub.3 and NiFe. In addition, it is desirable that the CMP endpoint be detected in situ and in real time; that is, not rely on external measurements such as the current drawn by shaft motor 16 or on extrapolation from previous layer thickness measurements.